1. Field of the Invention
The present invention relates to a radar apparatus that uses a frequency-modulated continuous wave (FW-CW) as a transmit wave and, more particularly, to a radar apparatus that performs scanning for receiving reflected waves of a transmitted radiowave by digital beam forming (DBF).
2. Description of the Related Art
Various radar apparatuses, of a type that performs scanning on receive by using DBF, have been developed and are known in the prior art. Generally, the basic configuration of this type of radar apparatus employs a single transmitting antenna and a plurality of receiving antennas, and a radiowave is transmitted from the transmitting antenna and reflected waves of the transmitted radiowave are received by the plurality of antennas.
However, the radar apparatus of this configuration requires as many receivers as there are receiving antennas, and a large number of receivers must be provided if the scanning accuracy is to be enhanced. This has presented the problem that as the number of receivers increases, the weight and size of the apparatus increases and, what is more, a large amount of power is needed.
To solve the above problem, radar apparatuses that achieve size and weight reductions are proposed, for example, in Japanese Unexamined Patent Publication Nos. S63-256879, H11-311668, and H11-160423. Such radar apparatuses are constructed so that a plurality of receiving antennas are connected to a single receiver via a switch. Alternatively, a plurality of receiving antennas are divided into several groups, that is, a receiving antenna array comprising a large number of receiving antennas is divided into subarrays each with four receiving antennas, for example, and one receiver is provided for each subarray of four receiving antennas, the four receiving antennas being connected to the receiver via a switch. When receiving reflected waves of a transmitted radiowave, the plurality of antennas are sequentially switched for connection to the receiver. In this way, radar signals received at the respective receiving antennas can be obtained in a time-division fashion.
With this configuration, the number of receivers can be reduced to one or to less than the number of receiving antennas, thus achieving a reduction in the size and cost of the apparatus.
Here, the radiowave used by the radar apparatus is a radiowave in a high-frequency band such as a 76-GHz band. Accordingly, the signals handled in the transmission paths from the receiving antennas to the receiver are also high-frequency band signals. The number of inputs on a switch that can switch such high-frequency signals is generally two or three.
Accordingly, when switching between four or more receiving antennas, it is practiced to use a plurality of switches. For example, unit switches, each being a single-pole double-throw (SPDT) or a single-pole three-throw (SP3T) switch, are combined in tree type to achieve multiple switching. Here, a high-frequency switch of planar circuit type, such as a monolithic microwave integrated circuit (MMIC) or a hybrid integrated circuit (HIC), is used as the unit switch.
However, when the switches are connected in multiple stages, signal attenuation increases as the signal passes through each switch, and hence the problem that, as the number of stages of switches is increased to reduce the number of receivers, reception sensitivity decreases, appears.
In view of this, a radar apparatus relatively simple in configuration and capable of preventing the degradation of reception sensitivity is proposed, for example, in Japanese Unexamined Patent Publication No. 2000-155171. While the radar apparatuses earlier described comprise a single transmitting antenna and a plurality of receiving antennas, the radar apparatus disclosed in this patent document employs a plurality of transmitting antennas, which are used by being switched from one to another, and thereby reduces the number of receiving antennas as well as the number of switches used to switch between the receiving antennas. It is claimed that with this configuration, reception sensitivity can be increased, while reducing the cost of the apparatus by reducing the number of antennas and switches.
This radar apparatus comprises four transmitting antennas A1 to A3 and two receiving antennas A4 and A5 arranged in a straight line on the same plane, the total number of antennas being smaller than that in any of the prior known radar apparatuses. It is claimed that this radar apparatus configuration makes the fabrication of the radar apparatus easier, can reduce the cost and, in applications such as automotive radar, can make the overall shape of the radar suitable for mounting on a vehicle.
According to the above radar apparatus, the attenuation of received signals due to switches can be reduced, and a number of channels larger than the number of antennas can be achieved with fewer antennas. According to the above radar apparatus, more channels can be achieved in DBF than there are antennas and, to obtain a narrower beam of higher directivity and, for example, to achieve nine channels, the number of antennas can be reduced to six, compared with 10 required in the prior art configuration, but when the antennas are mounted to construct the radar apparatus, as the antennas are arrayed in a single row, the six antennas require a mounting space equivalent to ten antennas.
On the other hand, when such a radar apparatus is mounted as an electronic apparatus, for example, on an automobile or the like, the position where it can be mounted so as to transmit the radiowave forward is limited, and the available mounting space is quite restricted. The radar apparatus used in such an environment must be made as small as possible. Accordingly, while the number of antennas can be reduced, the above radar apparatus still leaves much to be desired when it comes to size reduction. Furthermore, from the standpoint of vehicle driving safety, it is desired to further increase the performance of the radar apparatus for recognizing targets ahead of the vehicle, and it is also desired to reduce the cost of the apparatus.
Accordingly, it is an object of the present invention to provide a radar apparatus that can achieve a size reduction, a performance increase, and a cost reduction while achieving multiple channels, by minimizing the number of antennas necessary to perform scanning by digital beam forming (DBF) for reception of the reflected waves of a transmitted radiowave, and that can also achieve enhancements in the speed and accuracy of azimuth detection.